As a successor of a WCDMA (Wideband Code Division Multiplexing Access) system, an HSDPA (High-Speed Downlink Packet Access) system, and an HSUPA (High-Speed Uplink Packet Access) system, an LTE (Long Term Evolution) system has been considered and standardized by 3GPP (The 3rd Generation Partnership Project), which is a standardization organization of WCDMA.
Furthermore, as a successor of the LTE system, an LTE-advanced system is under consideration by 3GPP. The requirements for the LTE-advanced system are summarized in the non-patent document 1.
As one of the requirements in the LTE-advanced system, an agreement is reached that carrier aggregation is applied. When carrier aggregation is applied, a mobile station UE can receive downlink signals simultaneously using plural carriers or transmit uplink signals simultaneously using plural carriers. Each carrier used in carrier aggregation is referred to as a “component carrier”.
The plural component carriers are categorized into a primary component carrier as a main carrier and one or more secondary component carriers other than the primary component carrier.
When a mobile station UE performs communications always using the primary component carrier and the secondary component carriers, a problem arises that power consumption becomes higher in proportion to the number of component carriers. As used herein, communicating using the primary component carrier and the secondary component carriers includes usual data transmission and reception, cell search or measurement on the respective carriers, and radio link monitoring.
For example, the cell search includes establishing synchronization in downlink using downlink synchronization signals in a serving cell and an adjacent cell. Since cell search is the processing for detecting a destination cell while a mobile station UE is moving, the mobile station UE periodically needs to perform cell search. For example, the measurement includes measuring received power (more specifically, RSRP (Reference Signal Received Power) or the like) of reference signals in a serving cell and an adjacent cell. It should be noted that the combined processing of cell search and measurement may be referred to as “measurement”. The radio link monitoring includes measuring radio quality (more specifically, SIR (Signal-to-Interference Ratio)) of reference signals in a serving cell, determining whether the SIR is above a predetermined threshold, and determining that the serving cell is in out-of-synchronization when the SIR is below the predetermined threshold. The processing associated with cell search, measurement, and radio link monitoring and their performance definitions are described in non-patent documents 2 and 3, for example.
In order to address the problem of power consumption, it is considered that control of activation/de-activation is applied in the secondary component carrier, for example. For example, on a secondary component carrier in a de-activation state, the mobile station UE does not perform usual data transmission and reception and reduces the frequencies of cell search, measurement, and radio link monitoring, thereby saving the battery. The processing of de-activation on a secondary component carrier is performed when the amount of data to be communicated is small, for example.
In addition, in the LTE system, a measurement gap is defined in order to perform measurement on a carrier with a different frequency or a carrier for a different radio communication system (non-patent document 4). The length of the measurement gap is defined as 6 ms and its periodicity is defined as 40 ms or 80 ms, for example. During the measurement gap, the mobile station UE suspends communications in a serving cell and performs measurement of a carrier of a different frequency or a carrier of a different radio communication system. In this case, communication with the serving cell is stopped, throughput of communication with the serving cell deteriorates.